URBAN EFFECTS ON GROUNDWATER SYSTEMS

Areal expansion of cities (urbanization) is one of the major modern geomorphic processes, and urbanization alters groundwater systems significantly. Construction of underground structures and utility systems creates secondary porosity and permeability fields similar to those observed in karstic systems. Commonly we find that permeability increases along utility trenches by around 3 orders of magnitude and we infer higher increases. This creates considerable uncertainty where evaluation of solute plumes is required. Where situated below the water table, the utility network ("reticulation") may serve as drains for the groundwater system; above the water table, they become sources of concentrated recharge. Increased impervious cover reduces increases flood flows and decreases direct infiltration, but indirect recharge from leaky utility systems and irrigation return flow from watering of parks and lawns generally more than compensate. Data for cities worldwide confirm that urbanization increases recharge. Data for leakage from water mains range from 5 to over 60%. Leakage from sewer systems is commonly estimated at 5%, but can be much higher. The effects of indirect recharge are more significant in arid and semi-arid climates. In Austin, Texas, we estimate conservatively that recharge has nearly doubled with urbanization. Rising water tables occur where surface waters serve as the cities' major water resource. When coupled with changes in water quality, local climate systems, and surface water courses caused by urbanization, urban groundwater systems are evolving very rapidly. These effects should be considered in land management and urban design.